1. Field of the Invention
This invention relates to a noise cancellation system, and in particular to a noise cancellation system for inclusion in a sound reproducing device, and to a method of operation of such a noise cancellation system that is able to detect when the sound reproducing device is in a primary operating position.
2. Description of the Related Art
Noise cancellation systems are known, in which ambient noise is detected by means of one or more microphone, and the resulting ambient noise signal is applied to signal processing circuitry to generate a corresponding ambient noise cancellation signal. The ambient noise cancellation signal is then applied to a speaker, which is typically also being used to play wanted sounds to the user. Systems of this type are typically used in sound reproducing devices that are intended to be used close to the ear of the user, such as headphones, earphones or handsets, and the wanted sounds might be music, or speech, for example.
Effective noise cancellation is achieved when the signal processing circuitry generates an ambient noise cancellation signal that, when played through the speaker, generates a sound that is equal in magnitude but opposite in phase to the ambient sounds, as they reach the ear of the user. Thus, the signal processing circuitry performs a signal processing operation that must take account amongst other things of the difference between the ambient noise that is detected by the noise microphone, or microphones, and the ambient noise that reaches the ear of the user. In the case of headphones or earphones, this difference might be relatively constant, because the headphones or earphones are usually worn in a fixed position. However, in the case of a handset, this difference can vary quite substantially, because the user can hold the handset against his head in different ways.
Although noise cancellation can be effective when the desired signal processing is provided, ineffective noise cancellation can appear as an additional noise source, and can therefore be distracting to the user of the device. In addition, noise cancellation requires a power source, such as a battery, and generating ineffective noise cancellation signals is wasteful of the battery.
GB-2441835A discloses a noise cancellation system, in which an error microphone is positioned in the sound reproducing device, in order to detect the sounds that reach the ear of the user. The signals from the error microphone are then used to adapt the signal processing circuitry, which can then be used to generate effective noise cancellation. However, when the sound reproducing device is positioned away, or completely removed, from the user's ear, the noise cancellation system is unable to provide effective noise cancellation.
It is known to provide a proximity detector in a mobile phone handset to detect when the handset is against the users' head. For example, the proximity detector can be based on an infrared source and detector.